A compound semiconductor wafer manufacturing method is as follows. A cylindrical single-crystal ingot is grown from a polycrystalline raw material using single-crystal-growing equipment such as an LEC device, an HB device, or a VB device. Single-crystal ingots that do not turn out cylindrical are ground into a cylinder. The cylindrical single-crystal ingot is sliced through a plane perpendicular to its axis, rendering an as-cut wafer of constant thickness. The ridgeline along the wafer circumferentially is then thinly shaved obliquely. This is called chamfering or beveling. This wafer is treated by etching—lapping—etching processes. The etching process at the beginning removes manufacturing distortions; the lapping process makes the thickness a predetermined depth; the etching process at the end removes distortions due to lapping. The initial etching process can be left out. A beveled wafer of constant thickness is thus produced. These steps will herein be referred to as a polishing preprocess procedure.
Subsequently, to make an as-cut wafer into a mirror wafer, a number of processes are carried out. Process flow according to a conventional method will be explained using FIG. 1 (left side) and FIG. 2. Herein several processes will be collectively termed a procedure. A single procedure is a set of a plurality of processes. How they are arranged is not fixed; herein they are classified according the inventors' preference. Likewise, the process categories themselves differ depending on the objective and on the persons involved.
Explanations are more often made without segregating processes from procedures. Moreover, while “process” is often used as a concept that is broader than “procedure,” such will not be the case in this specification. As set out by the present invention, process as a minor category will for convenience of explanation be distinguished from procedure as a major category.
The procedures herein are a group of five as in FIG. 1: a polishing-preprocess procedure (a); a polishing procedure (b); a cleaning procedure (c); an inspection/measuring procedure (d); and a final cleaning procedure (e). A single procedure includes a plurality of processes. To the end of the word “process,” “operation,” “step,” or nothing at all, will be affixed. Procedure and process should not be confused.
(a) Polishing-Preprocess Procedure (Preparatory Procedure for Polishing)
A. Lapping—flattens either surface of as-cut wafers to a predetermined thickness. Etching to remove slicing-procedure distortions is sometimes done prior to that. Herein the etching process at the beginning is omitted. The wafers are then beveled to shave the ridgeline diagonally smooth.
B. Etching—removes processing distortions due to lapping.
(b) Polishing Procedure (Procedure for Polishing Wafers Using Polishing Equipment)
C. Applying paste to non-polish surface—the reverse surfaces of the wafers are heated and a flowing wax (the paste) is spread on them to paste them to a polishing plate. This is because without the wax the wafers will not adhere to the polishing plate. Applying the wax smudges the wafers, and the smudges cannot easily be removed. This is a problem.
D. Pasting to polishing plate—the reverse surfaces of the wafers, smeared with the wax, are pasted to the polishing plate. Four wafers are pasted to a single polishing plate in FIG. 2. The exteriorly exposed surface is the obverse surface. The polishing plate is a jig that is a disk onto which a shaft is fixed. Some is for single-wafer, and some is for multiple-wafer, depending on the dimensions. When the temperature lowers, the wax hardens, and the wafer is held fast to the polishing plate.
E. Mirror Polishing—the polishing face of the polishing plate is set onto a broad platen onto which a polishing cloth is bonded, and the wafers are pressed against the platen by putting pressure on the shaft. The platen is rotated while a polishing liquid is fed in. Herein, a single polishing plate on a single platen is illustrated, but a plurality of polishing plates can be set onto the polishing cloth and polished simultaneously. The obverse surface is thereby mirror-polished smooth and flat.
F. Drying—the polished wafers, attached as they are to the polishing plate, undergo as in FIG. 2 a simple wash using purified water. Thereafter they are dried while still attached to the polishing plate. Drying is done in the atmosphere (clean-room), and this operation actually gives rise to many problems. Because there is oxygen within the atmosphere, the obverse surface oxidizes. Due to the ambient atmosphere, the obverse surface alters. More than anything else, debris (particles) within the air fall in due course and adhere to the wafers. Fouling matter on the obverse surface of a damp-dried wafer is not easily removed. In short, on account of the post-polish drying, deterioration of the obverse surface in that particles adhere to it, and it alters and oxidizes, is brought about.
G. Peeling off from the polishing plate—the wafers are firmly fixed to the polishing plate by the wax. The polishing plate is heated to make the wax run, and the wafers are unstuck from the polishing plate using a spatula. While the obverse surface has become fine and mirror-like, the reverse surface is left smeared with wax.
H. Atmospheric storage—as indicated in FIG. 2, a plurality of wafers is housed into a cassette, which is stored atmospherically.
The foregoing C.—H. are the polishing procedure. What is thereby obtained is wafers whose obverse surface is polished mirror-like, and on whose reverse surface wax (organic compound) is adhered.
(c) Cleaning Procedure (Using Organic Solvent, Alkali, and Purified Water to Wash Away Wax Smeared on Wafer Reverse Surface, and Particles Adhered to the Obverse Surface)
I. Organic solvent wash—the wafers are washed in an organic solvent such as acetone or alcohol. Clears away wax on the reverse surface. Also gets rid of smudges adhering to the obverse surface. Since the wafers are adhered with wax, this process is indispensable.
J. Alkali wash—organic solvent, obverse-surface oxidization film, and smudges are taken away using caustic soda or the like.
K. Purified water wash—organic solvent, alkali, and smudges are taken away using purified water.
L. Organic solvent vapor drying—the purified water or other cleaning liquid is globally subjected to rapid vaporization and the wafer is dried.
Processes I.—L. are the cleaning procedure. This procedure may be carried out step-by-step with the plurality of wafers put into a cassette. Through this procedure the wafers turn out as mirror wafers having a clean obverse surface.
(d) Inspection/Measuring Procedure (Inspection/Measuring According to Specifications from the Shipping Destination)
M. Obverse surface inspection/gauging—microscopic observation of the wafer obverse surface and gauging of particle density are performed. For example, the number of particles whose diameter is a set value or more is counted. Mirror wafers that meet the specifications of the shipping destination (device manufacturer) will thereby be produced. Wafers passing these tests undergo a final cleaning as needed and are shipped out.
(e) Final Cleaning Procedure (Final Manufacturer-End Cleaning of Wafers that have Passed Inspection)
N. Alkali wash—organic solvent, organic matter and other contaminants adhering to the obverse surface are taken away using alkali.
O. Purified water wash—organic solvent, alkali, and smudges are taken away using purified water.
P. Organic solvent vapor drying—the purified water or other cleaning liquid is globally subjected to rapid vaporization and the wafer is dried.
In this way mirror wafers are fabricated, and are shipped in that state to the device maker. As such the device maker cannot use them as substrates for thin-film growing (layering). This is because in the course after being packaged and until reaching the device maker they become oxidized, contaminated, and dirtied with adhering particles. Therefore, prior to thin-film growing the wafers have had to be etched and cleaned on the manufacturer's end.
Prior Art I., Japanese Pub. Pat. App. No. H11-204471, “Compound Semiconductor Wafer Manufacturing Method,” provides for oxidizing using ozone after polishing a semiconductor wafer and before drying and cleaning. An oxide layer that is 1.5 nm or more in thickness is said to be satisfactory. Fashioning the oxide layer leads to debris riding onto the oxide layer on the wafer. Subsequently cleaning using alkali to get rid of the oxide layer and at once to get rid of the debris is then provided for. This means that the obverse surface will become clean, since removing the oxide layer also removes the debris thereon.
Nevertheless, in this situation in which a thick oxide layer is thus cleared away with alkali, roughness appears in the obverse surface of the wafer, such that it is no longer a mirror wafer. Furthermore, it can happen that a wafer is oxidized after being soiled with debris, such that the wafer will not be oxidized beneath where the debris is. Since the unoxidized areas will not be taken away by the alkali wash, the debris will remain as it is in form. Though a compound semiconductor wafer is oxidized naturally in the atmosphere, a thinner oxide layer is better. Deliberately oxidizing, a concept that might crop up interestingly enough, would not attain the anticipated objectives.
The procedures for manufacturing a clean, mirrorlike wafer are completed according to the polishing-preprocess procedure (a) through the final cleaning procedure (e). There are problems as follows with the procedures described above.
1. Particles adhere to the wafer obverse surface in the drying process F. that follows polishing. Foreign matter cannot easily be taken off a wafer when it is wetted with water. Repeatedly cleaning any number of times reduces the number of particles to a certain extent. But the adhering particles cannot be gotten rid of entirely.
2. In the post-polishing drying process F. the wafer obverse surface alters because it is exposed to the atmosphere.
3. Since the wafers are attached to the polishing plate with wax, organic matter remains behind. Residual organic matter on the substrate obverse surface gives rise to negative influences when thin films are built on it later.
4. In order to get rid of the wax, the wafers are cleaned using an organic solvent. Completely taking off the wax using an organic solvent is difficult, however. Even if the cleaning liquid is increased, and even with multi-stage cleaning, the organic matter cannot be gotten rid of completely.
5. Excessive use of organic solvents worsens the work environment. Disposing of the liquid waste from the organic solvent also becomes a problem.
6. Organic and foreign matter remaining behind on the wafer obverse surface have led to abnormal growth when the thin films are grown at the device maker.
7. In order to prevent abnormal growth, etching and cleaning have to be carried out on the device maker end (at the shipping destination).